A big win for mobile startup Loopt: Verizon Wireless, the second-biggest U.S. wireless carrier, will offer its 66 million subscribers access to Loopt's mobile app next month. What is Loopt? A souped-up mapping/social networking program that lets you share your location with your friends, see where they are on a map, message them, share photos, etc.

Will it catch on? "Location-based services" like Loopt have been hyped for years, but have been a big disappointment so far. "Hundreds of thousands" of Sprint Nextel (S) subscribers have signed up for the service, according to the Wall Street Journal. But it's been free so far, and "signing up" doesn't necessarily mean "using regularly." And even if "hundreds of thousands" means 999,999, that's still less than 2% of Sprint's subscriber base.

In theory, adding Verizon Wireless to Loopt's network -- which more than doubles the number of potential subscribers -- could help it take off. But some hurdles remain. For instance, subscribers need a GPS-enabled phone to sign up -- ruling out many subscribers. (We asked, and Verizon didn't specify how many of its phones would be compatible at launch.) And the company plans to charge $4 a month, or $48 a year, for the service, which will limit uptake.

More important, Loopt might not work across mobile networks when it launches on Verizon Wireless, the carrier confirms. Update: A Loopt rep tells us the service will be interoperable at launch. Good news. That means that if you're a Verizon subscriber, you might not be able to interact with a Sprint subscriber, which will severely limit Loopt's usefulness. (If you recall, hardly anyone used text messaging in the U.S. until it worked across multiple carriers.) Good news: Verizon realizes this. "Interoperability will be a big driver for these kinds of services," a spokesman tells us by email.

So will Loopt be a hit? Maybe. But we think the killer version of this app will end up integrated into existing platforms. It wouldn't be tricky for someone bigger like Google (GOOG) to add similar (and free) features to their mobile Google Maps app, or for Facebook to add a map and location info to their (free) mobile site, which already knows who you're "friends" with.